drm: bridge: simple-bridge: Delegate operations to next bridge

Instead of poking into the DT node of the next bridge for its DDC bus
and implementing the .get_modes() and .detect() connector operations
manually, retrieve the next bridge in the chain and delegate these
operations to it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200526011505.31884-7-laurent.pinchart+renesas@ideasonboard.com